The present invention relates generally to a microscope objective and a single objective type binocular stereomicroscope system, and more particularly to an objective for use on a single objective type binocular stereomicroscope system which, albeit having a parfocal length thereof shorter than a focal length thereof, reduces various aberrations as much as possible, and forms an image improved in terms of flatness, and a single objective type binocular stereomicroscope system using such an objective, which can be well manipulated while its viewing position remains invariable.
In regard to a generally available single objective type stereomicroscope, its (parfocal) length from an object surface to a location where its objective is mounted thereon varies depending on the type, and magnification of the objective. Especially at a low magnification, lower than 1.times., there is a focal length increase. The parfocal length of an objective for use on the conventional objective type binocular stereomicroscope is equivalent to or longer than the focal length thereof. When an object is observed using a low-power objective having a magnification of 0.5.times. for instance, that parfocal length is as long as about 60 mm to about 90 mm (typically 77 mm as shown in FIG. 15(b)), as compared with a viewing position of 1.times. as shown in FIG. 15(a). In FIGS. 15(a) and 15(b), it is noted that reference numeral 1 represents a lens housing, 2 an eyepiece, 3 represents an objective having a magnification of 1.times., 4 represents an objective having a magnification of 0.5.times., 5 represents a lens barrel, 6 represents a parfocal length of the objective of 1.times., 7 represents a parfocal length of the objective of 0.5.times., and 8 represents a viewing position difference between when observing an object using the objective of 1.times. and when observing the object using the objective of 0.5.times..
JP-B-60-227214 describes a binocular stereomicroscope equipped with a revolver unit. However, this stereo-microscope is difficult to manipulate because when the objective of 1.times. is changed to the objective of 0.5.times., focusing control must be carried out over a length as long as about 60 mm to about 90 mm corresponding to the aforesaid viewing position difference.
The aforesaid problem may be solved by making the parfocal length of a low-magnification objective for a single objective type binocular stereomicroscope nearly equal to that of other objective, e.g., one having a focal length shorter than that of the aforesaid objective and a magnification of 1.times.. For a microscope objective it is common knowledge to have an invariable parfocal length; however, this is not the case for a conventional stereomicroscope. A chief reason for this is that, in view of aberration correction, difficulty is experienced in designing a low-magnification stereomicroscope objective such that its parfocal length is shorter than its focal length. To shorten the parfocal length without varying the focal length, it is required that a principal point position within the objective or on the object side be transposed to the image side. To achieve the aforesaid object, therefore, it is required to increase the power of each lens group within the objective. Since light rays propagating through the stereomicroscope objective are subject to decentration, a sample surface is viewed in different ways in the horizontal direction. In addition, the objective is often used in combination with a zooming optical system. The range of view, and NA (numerical aperture) of the objective vary with a magnification change of the zooming optical system. That is, there is much difficulty involved in the elimination of longitudinal chromatic aberration at high magnifications and the elimination of chromatic aberration of magnification at low magnifications.
It is noted that it is relatively easy to bring the parfocal length of a standard objective in alignment with that of an objective having a shorter focal length and a higher magnification because it is possible to transpose the mounting position on the image side by inserting a spacer or the like between the objective and its lens housing while taking advantage of the fact that a light flux leaving the objective lens is of afocal nature.